The overall specific aims are to define the photoperiod conditions which permit expression of endogenous circannual rhythms of reproductive activity (anestrus versus estrous cycles as defined by blood progesterone) and prolactin secretion and to define the neural structures involved in generation and control of these rhythms using the sheep as the experimental model. Specific aims are: 1. To complete a study in progress testing the hypothesis that endogenous rhythms of reproductive activity in sheep are expressed only under certain constant photoperiods. Expression of circannual rhythms of reproductive activity and prolactin secretion will be compared in ewes housed under 12L:12D versus 8L:16D photoperiods. 2. To test the hypothesis that the effect of a specific signal (photoperiod regimen) on expression of endogenous circannual rhythms depends on the stage of the annual rhythm at which that signal is applied. Ewes will be placed under 12L:12D at 4 distinct stages of their annual cycle. Subsequent patterns of reproductive activity and prolactin secretion will be monitored. 3. To test the hypothesis that expression of an endogenous rhythm of reproductive activity is masked in ovary-intact ewes kept under a 12L:12D photoperiod. Comparison will be made of temporal changes in onset and cessation of estrous cycles in intact ewes versus changes of LH in ovariectomized estrogen-treated ewes housed under 12L:12D and 8L:16D photoperiods. 4. To test the null hypothesis that removal of the olfactory bulbs will not affect the ability of photoperiod changes to alter LH and prolactin secretion in ewes. The effect of olfactory bulbectomy on basal secretion of LH and FSH and on responses of these hormones to changes in photoperiod will be monitored. 5. To test the null hypotheses that lesions of the suprachiasmatic nucleus (SCN), lesions of the anterior hypothalamic area (AHA), or olfactory bulbectomy will not affect expression of circannual rhythms of reproductive activity and prolactin secretion in ewes kept under a constant photoperiod. In 3 separate experiments ewes will be subjected to either SCN lesions, AHA lesions, olfactory bulbectomy, or sham surgery and then housed under 8L:16D to determine if expression of the putative endogenous rhythm is altered.